Summary This proposal describes a 5-year research plan focusing on the novel concept that the skin can serve as a site to induce systemic immunomodulation, which could be exploited therapeutically to treat diseases with a T cell- mediated systemic inflammatory component such as atherosclerosis. Current treatments for systemic inflammatory diseases include oral immunosuppressive regimens and biologics that are administered parenterally. Although these treatments are effective in some inflammatory diseases, they are associated with significant systemic side effects (oral immunosuppressives), are expensive (biologics), and are cumbersome to administer (biologics). Thus, developing new therapeutic strategies for inflammatory diseases such as atherosclerosis that cause minimal side effects, are cheap, and are easy to administer is of critical importance. We provide preliminary data that the skin has a surprising ability to affect systemic immune responses. The topical application of the Vitamin D3 analog MC903 induces the cytokine thymic stromal lymphopoietin (TSLP) from skin keratinocytes, which enhances systemic basophil hematopoiesis and shifts systemic immune responses to Th2. Moreover, we demonstrate that topical MC903 treatment induces systemic Treg expansion in a TSLP-dependent manner. Although the effects of MC903-treated skin on Tregs and Th2 responses are both dependent on TSLP, the effect on Tregs and Th2 skewing is separable by targeting downstream effector molecules. Importantly, topical MC903 treatment significantly attenuates atherosclerotic lesion size by ~65% in the Apo E knockout (KO) mouse model of atherosclerosis. We also have preliminary data in human patients demonstrating significant increases in circulating regulatory T cells 7 days after topical application of MC903. Based on these preliminary data, we hypothesize that MC903-treated skin systemically increases Tregs and Th2 responses, which is useful for the treatment of atherosclerosis. Our research plan (first two aims) starts by investigating the cellular and molecular mechanisms that are involved in Treg expansion by MC903-treated skin of mice and to preliminarily test whether MC903 acts similarly in humans. Our third Aim will focus on the mechanisms by which MC903-treated skin affects the pathogenesis of atherosclerosis. I have recruited several key personnel to increase the likelihood of success in accomplishing the goals of this proposal. Dr. Daniel Rader is a cardiologist, who is a world-renowned expert in hyperlipidemia and in mouse models of atherosclerosis. Dr. Demehri will be our collaborator for clinical studies to investigate the effects of MC903 treatment on circulating Tregs in human subjects. We believe that the studies outlined in this proposal will lead to paradigm-shifting impact on understanding of how the skin regulates systemic immune homeostasis. This effect could be exploited for the therapeutic attenuation of systemic inflammation in atherosclerosis, which could be extended to other diseases with a similar T cell-mediated inflammatory component.